My two colleagues, Sheila, Sue and I went to visit my brother who was one of the Head Rangers in Yosemite at the time. We wanted to hike up Half Dome as none of us, my brother included, had done it before.
We started early (9AM) and brought lots of water and food for lunch. It's a 17 mile round trip and it took us the better part of four hours to reach the base of the summit of half Dome. We ran out of water and stupidly did not bring a filter with us. We had lunch at the bottom of the cables and drank the last of our water.
My brother had horrible blisters on his heels and begged of climbing to the top. After going about 50 feet up my friend Sheila got vertigo and decided to stay behind. That left me and Sue. By far the hairiest part was the cables that you grip onto for dear life to go up and come down on.
There were people having freak outs ahead of us, who were too scared to go up or down. They blocked the way and so you had to go outside the cables to get around them. We finally made it to the top and found snow up there. We took as much as we could and filled or camelback's for the trip back.
We still ran out of water and my brother the ranger drank out of the merced river, with no ill effects.